On Friday, 12. September 2025 at 10 AM, the doctoral defense of Simon Peters will take. The title of his dissertation is "Computational Multiscale Modeling of Low-Carbon Concretes at Elevated Temperatures".

Abstract:
Despite decades of research, the underlining mechanism of explosive concrete spalling at elevated temperatures remains unknown. This thesis proposes a fully coupled numerical chemo-thermo-hygro-mechanical model, advanced through a micromechanical framework, providing a deeper understanding of the multiphysical nature of explosive spalling.

Key contributions include the development and validation of the micromechanical framework to analyze the binder-specific dehydration behavior, chemically induced material evolution and microstructure. This framework enhances applicability and reduces experimental calibration requirements for the well-established multiphysical macroscale model, particularly for concretes based on CO2-reduced cements.

By means of virtual parametric studies, the main findings of the thesis are: i) The binder-specific dehydration behavior of CO2-reduced cement pastes is not the primary driving mechanism behind fire-induced concrete spalling. ii) Aggregates characterized by high thermal conductivity can lead to a significant increase (even more than 35%) in pore pressure when compared to aggregates with lower thermal conductivity. iii) The dense microstructure is the primary factor driving the susceptibility of concretes containing CO2-reduced cements to fire-induced concrete spalling compared to ordinary concretes. iv) The moisture clog theory is not supported.

Written by Roger A. Sauer the new Open access publication "A curvilinear surface ALE formulation for self-evolving Navier-Stokes manifolds – stabilized finite element formulation" is published now in the journal 'Computer Methods in Applied Mechanics and Engineering' by Elsevier.

Abstract:
This work presents a stabilized finite element formulation of the arbitrary Lagrangian-Eulerian (ALE) surface theory for Navier-Stokes flow on self-evolving manifolds developed in Sauer [1] . The formulation is physically frame-invariant, applicable to large deformations, and relevant to fluidic surfaces such as soap films, capillary menisci and lipid membranes, which are complex and inherently unstable physical systems. It is applied here to area-incompressible surface flows using a stabilized pressure-velocity (or surface tension-velocity) formulation based on quadratic finite elements and implicit time integration. The unknown ALE mesh motion is determined by membrane elasticity such that the in-plane mesh motion is stabilized without affecting the physical behavior of the system. The resulting three-field system is monolithically coupled, and fully linearized within the Newton-Rhapson solution method. The new formulation is demonstrated on several challenging examples including shear flow on self-evolving surfaces and inflating soap bubbles with partial inflow on evolving boundaries. Optimal convergence rates are obtained in all cases. Particularly advantageous are C1-continuous surface discretizations, for example based on NURBS.

'A curvilinear surface ALE formulation for self-evolving Navier–Stokes manifolds: general theory and analytical solutions' is a new open-access paper written by Roger A. Sauer. It is published in the 'Journal of Fluid Mechanics' by Cambridge University Press.

Abstract:
A new arbitrary Lagrangian–Eulerian (ALE) formulation for Navier–Stokes flow on self-evolving surfaces is presented. It is based on a general curvilinear surface parameterisation that describes the motion of the ALE frame. Its in-plane part becomes fully arbitrary, while its out-of-plane part follows the material motion of the surface. This allows for the description of flows on deforming surfaces using only surface meshes. The unknown fields are the fluid density or pressure, the fluid velocity and the surface motion, where the latter two share the same normal velocity. The corresponding field equations are the continuity equation or area-incompressibility constraint, the surface Navier–Stokes equations and suitable surface mesh equations. Particularly advantageous are mesh equations based on membrane elasticity. The presentation focuses on the coupled set of strong and weak form equations, and presents several manufactured steady and transient solutions. These solutions are used together with numerical simulations to illustrate and discuss the properties of the proposed new ALE formulation. They also serve as basis for the development and verification of corresponding computational methods. The new formulation allows for a detailed study of fluidic membranes such as soap films, capillary menisci and lipid bilayers.

On Monday, 11th August 2025, a research group from Wuhan University, China visited us - the Institute for Structural Mechanics, Ruhr University Bochum, Germany - again. Since January 2021 we both have been actively collaborative in the research project "Multiscale Tests, Simulation, Optimization and 3D-Printing of UHPFRC Materials and Structures", a project between Prof. Günther Meschke (Institute for Structural Mechanics, RUB) and Prof. Zhenjung Yang (Wuhan University, China), that has been embedded in a mobility programme. Now, after 4 years of research, it comes to an end.

The joint research focused on the development of innovative solutions in the field of 3D-printed ultra-high-performance fibre-reinforced concrete (UHPFRC). The Wuhan University team performed CT scans and provides high-resolution CT images. We, the team from the Institute for Structural Mechanics, used these CT images to create high-resolution digital 3D models from which directly numerical models are automatically generated. These numerical models allowed us to analyze the failure mechanisms of 3D-printed fibre-reinforced concrete on the meso-scale very precisely.

The completion of this joint research project represents an important milestone in our collaboration. The results achieved provide valuable insights into UHPFRC materials and structures and contribute to the current knowledge in this area.
Moving forward, there are several potential directions for further research. The findings from this project can serve as a foundation for additional studies or applying these results in practical settings or expanding the scope to include new variables.
This project has also paved the way for continued collaboration between our teams, allowing us to address more complex challenges in the future.

The open access paper titled "Computational hydro-thermal design of ground freezing in tunneling: Optimization of pipe layout and real-time prediction" has been witten by Rodolfo Javier Williams Moises, Yaman Zendaki, Ba Trung Cao, and Günther Meschke.
It is now published in "Tunnelling and Underground Space Technology" by Elsevier.

Abstract:
Artificial ground freezing is used in tunneling for temporary ground improvement mainly to control the groundwater flow and to provide excavation support. The principle of ground freezing is based on freeze pipes drilled into the ground where a coolant flows through the freeze pipes. In tunneling, artificial ground freezing is applied to form a closed arch of frozen ground around the tunnel. However, high groundwater seepage can delay the formation of this frozen arch, or even prevent it entirely, which can lead to unsafe temporary frozen ground support. In this work, we propose a computational design strategy to systematically reduce the freezing time by optimizing the layout of freeze pipes. The strategy includes a computational model able to simulate the freezing process for various pipe arrangements, and the generation of a machine learning model of the freezing process trained by means of virtual, simulation-based data to allow real-time predictions. This surrogate model is constructed based on the combination of Proper Orthogonal Decomposition and Radial Basis Functions, while the freezing process itself is simulated with a hydro-thermal finite element model. Using the surrogate model, possible pipe layouts are rapidly evaluated and optimized in real-time by means of the Particle Swarm Optimization approach, identifying the optimal arrangement of freezing pipes for given groundwater flow conditions which lead to the shortest freezing time. The computational design framework for optimization of freeze pipe layout is integrated into an in-house user-friendly software which performs real-time predictions within a parametric design space.

The open access paper is available here: https://doi.org/10.1016/j.tust.2025.106875